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The basics of metal fatigue
Common sense tells us that nothing that is used lasts forever. The more you use something, and the harder
you use it, and the worse the conditions you use it in, the shorter its life.
Fatigue is the term used to describe accumulated damage to a part caused by repeated loading. To cause
fatigue damage, the load the part receives must be great enough. A crude, often-used example is bending a
paper clip back and forth (repeated loading) until it breaks. This simple definition will help you understand
that fatigue has nothing to do with time or age. A bicycle in a garage does not fatigue. Fatigue happens only
through use.
So what kind of “damage” are we talking about? On a microscopic level, a crack forms in a highly stressed
area. As the load is repeatedly applied, the crack grows. At some point the crack becomes visible to the
naked eye. Eventually it becomes so large that the part is too weak to carry the load that it could carry
without the crack. At that point there can be a complete and immediate failure of the part.
One can design a part that is so strong that fatigue life is nearly infinite. This requires a lot of material and a
lot of weight. Any structure that must be light and strong will have a finite fatigue life. Aircraft, race cars,
motorcycles all have parts with finite fatigue lives. If you wanted a bicycle with an infinite fatigue life, it would
weigh far more than any bicycle sold today. So we all make a tradeoff: the wonderful, lightweight
performance we want requires that we inspect the structure.
A few things to think about
• ONCE A CRACKS STARTS IT CAN GROW AND GROW
FAST. Think about the crack as forming a pathway to
failure. This means that any crack is potentially dangerous
and will only become more dangerous.
SIMPLE RULE 1 :
If you find crack, replace the part.
• CORROSSION SPEEDS DAMAGE. Cracks grow more
quickly when they are in a corrosive environment. Think
about the corrosive solution as further weakening and
extending the crack.
SIMPLE RULE 2 :
Clean your bike, lubricate your bike, protect
your bike from salt, remove any salt as
soon as you can.
• STAINS AND DISCOLORATION CAN OCCUR NEAR A
CRACK. Such staining may be a warning sign that a crack
exists.
SIMPLE RULE 3 :
Inspect and investigate any staining to see
if it is associated with a crack.
• SIGNIFICANT SCRATCHES, GOUGES, DENTS OR
SCORING CREATE STARTING POINTS FOR CRACKS.
Think about the cut surface as a focal point for stress (in
fact engineers call such areas “stress risers,” areas where
the stress is increased). Perhaps you have seen glass cut?
Recall how the glass was scored and then broke on the
scored line.
SIMPLE RULE 4 :
Do not scratch, gouge or score any
surface. If you do, pay frequent attention to
this area or replace the part.
• SOME CRACKS (particularly larger ones) MAY MAKE
CREAKING NOISE AS YOU RIDE. Think about such a
noise as a serious warning signal. Note that a
well-maintained bicycle will be very quiet and free of creaks
and squeaks.
SIMPLE RULE 5 :
Investigate and find the source of any
noise. It may not a be a crack, but
whatever is causing the noise should be
fixed promptly.
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